function output = df2(lambda)

p0 = -6; % watch the sign!
gamma = 3;

output = (sech((sqrt(lambda)-sqrt(lambda +2*p0*gamma))/gamma))^2 * (1/sqrt(lambda) - 1/sqrt(lambda + 2*p0*gamma))/(2*gamma) - ... 
(9.8 + .5*sqrt(lambda)*gamma)/((9.8 + gamma*sqrt(lambda))^2);

end
